While landing on a 30 foot-wide runway in steady crosswind conditions, the aircraft encountered an unexpected gust of wind. Although the pilot tried to correct for the unfavorable wind conditions, his attempts to do so were not adequate to keep the aircraft from departing the side of the runway. After departing the runway surface, the aircraft encountered a swampy area, and nosed over in about nine inches of water. The pilot later realized that wind funneling between buildings and trees located alongside the runway had created a funnel effect that produced areas of crosswind gusts.